Prevent forced alpha in figureoptions.

[anntzer]

Forcing the alpha value sets it not only for the line color but also the
marker faces and edges (i.e., editing the alpha value of the markers
in the options editor had no effect). Instead, just remove any forced
alpha and use the alpha values that have been set in the editor.

A later idea may be to make alpha not override alpha values passed in
together with the color as quadruplets.

See discussion starting at #6383 (comment)

[tacaswell]

This has the effect of over-ridding any previously supplied user input, but that is ok as they are in the editor (and the way it was is also throwing away user input), so merging.

[tacaswell]

backported to v2.x as 5fb9017